Secondary transport as an efficient membrane transport mechanism for plant secondary metabolites

Plants produce a large number of secondary metabolites, such as alkaloids, terpenoids, and phenolic compounds. Secondary metabolites have various functions including protection against pathogens and UV light in plants, and have been used as natural medicines for humans utilizing their diverse biological activities. Many of these natural compounds are accumulated in a particular compartment such as vacuoles, and some are even translocated from source cells to sink organs via long distance transport. Both primary and secondary transporters are involved in such compartmentation and translocation, and many transporter genes, especially genes belonging to the multidrug and toxin extrusion type transporter family, which consists of 56 members in Arabidopsis, have been identified as responsible for the membrane transport of secondary metabolites. Better understandings of these transporters as well as the biosynthetic genes of secondary metabolites will be important for metabolic engineering aiming to increase the production of commercially valuable secondary metabolites in plant cells.     

Distribution of brominated compounds within the sponge<Emphasis Type="Italic"> Aplysina aerophoba</Emphasis>: coupling of X-ray microanalysis with cryofixation techniques

The major secondary metabolites of the sponge Aplysina aerophoba are brominated compounds. X-ray energy dispersive microanalysis was therefore used to locate secondary metabolites via the Br signal in energy emission spectra from sponge sections. To test the reliability of this method in the face of the loss or redistribution of metabolites during processing, we compared the results obtained by conventional aldehyde fixation with those obtained by cryofixation and cryosubstitution with and without cryoembedding. Bromine appeared to be concentrated in two sponge structures, viz. fibres and spherulous cells, when cryofixed material was examined. However, X-ray microanalysis failed to demonstrate the presence of bromine in spherulous cells in chemically fixed samples, showing the need for cryotechniques to avoid the loss of compounds. Cryofixation plus cryosubstitution methods performed best regarding structural preservation and the immobilization of metabolites. The presence of bromine in the spherulous cells suggests that this cell type is the producer of the secondary metabolites, as described for other sponge species. Nevertheless, the presence of bromine in sponge fibres indicates that they can accumulate metabolic substances, although we have been unable to assess whether the chemicals are in their original form or in a modified state within the fibres. A. aerophoba has both bacterial and cyanobacterial symbionts in its mesohyl; the absence of brominated compounds in them contrasts with previous findings in other sponges with prokaryote symbionts.     

Secondary metabolites as endogenous effectors of microbial cytodifferentiation

The present survey covers the regulatory role of microbial secondary metabolites and related compounds as endogenous signals of cell differentiation of the producing organisms. Several antibiotics have been shown to exert autoregulatory effects on differentiation-associated functions. The mechanisms of self-protection of the producing cells against the autotoxicity of secondary metabolites are discussed in terms of an integral part of the modulation of signal strength. As a further topic, the review deals with the hormone-like interference of particular metabolites with differentiating cells. Conclusive discussion concerns the potential use of microbial signal molecules either as tools for directed manipulations of product syntheses or as pharmaceutics.     

粘细菌次生代谢产物中大环类化合物的研究进展

粘细菌可以产生多种次生代谢产物,是继放线菌、芽胞杆菌之后的第3大类次级代谢产物产生菌。本文综述了粘细菌次生代谢产物中大环类化合物的研究进展。首先介绍了粘细菌中分离的主要大环类化合物种类、结构特征及来源菌株;然后分别从抗肿瘤、抗菌、抗病毒等几方面对其生物活性进行概述;最后对粘细菌次生代谢产物中大环类化合物的相关研究工作进行了展望。     

Secondary metabolites and plant/environment interactions: a view through Arabidopsis thaliana tinged glasses

Arabidopsis thaliana is a successful model plant for studying wide‐ranging topics including plant development, genetics and pathogen resistance. In addition, significant research has been conducted in the area of secondary metabolite biochemical genetics. The secondary metabolites in Arabidopsis include glucosinolates, terpenoids, phenylpropanoids, the alkaloid‐like camalexin, and other uncharacterized compounds. The genetic tools developed in studying secondary metabolite biochemistry are now being used to study how secondary metabolites control various biological processes. This includes compounds involved in plant/insect and plant/pathogen interactions, compounds preventing UV‐B damage, and compounds involved in hormone homeostasis. This review will describe what light Arabidopsis is shedding on the biological and ecological importance of specific secondary metabolites.     

Fungi of the genus Penicillium as producers of physiologically active compounds (Review)

Fungi of the genus Penicillium isolated from little studied habitats are able to synthesize both previously known and new physiologically active compounds with diverse structures. They include secondary metabolites of alkaloid nature, i.e., ergot alkaloids, diketopiperazines, quinolines, quinazolines, benzodiazepines, and polyketides. We discuss the use of profiles of secondary metabolites for taxonomy purposes. Studying the physicochemical characteristics of producers of biologically active compounds showed that the biosynthesis of alkaloids is initiated on the first days of cultivation and proceeds simultaneously with growth. The cyclic character of alkaloid accumulation was recorded related to the processes of alkaloid biosynthesis, excretion from cells, degradation in culture fluid, and consumption by cells. Synchronic variations in the concentrations of intracellular tryptophan and alkaloids are necessary for the regulation of the optimal quantity of tryptophan necessary for the culture.     

Biosynthesis of plant-specific stilbene polyketides in metabolically engineered Escherichia coli

Background  

Phenylpropanoids are the precursors to a range of important plant metabolites such as the cell wall constituent lignin and the secondary metabolites belonging to the flavonoid/stilbene class of compounds. The latter class of plant natural products has been shown to function in a wide range of biological activities. During the last few years an increasing number of health benefits have been associated with these compounds. In particular, they demonstrate potent antioxidant activity and the ability to selectively inhibit certain tyrosine kinases. Biosynthesis of many medicinally important plant secondary metabolites, including stilbenes, is frequently not very well understood and under tight spatial and temporal control, limiting their availability from plant sources. As an alternative, we sought to develop an approach for the biosynthesis of diverse stilbenes by engineered recombinant microbial cells.     

Histolocalization of chemotaxonomic markers in Brazilian Vernonieae (Asteraceae)

Vernonieae are well represented in South America by subtribe Lychnophorinae, with 104 Brazilian species, some of them rare and endangered. Lychnophorinae are well known for producing metabolites of high pharmaceutical and chemotaxonomic value. Despite the importance of secondary metabolites in Lychnophorinae, there is still little evidence regarding the secretory structures responsible for producing these compounds. Therefore, in this study we investigated 15 species representative of the main lineages of Lychnophorinae, with the following objectives: to identify where the chemotaxonomic markers of Lychnophorinae are secreted and stored and in which developmental stage of the organ these metabolites are found. Samples of stems and leaves were processed according to the usual techniques in plant anatomy. It was found that the main sites of secondary metabolite biosynthesis are glandular trichomes, epidermal cells and parenchyma tissues. Metabolites from glandular trichomes, especially sesquiterpene lactones (STL), are prevalent in the early developmental stages of organs. The metabolite compounds stored in parenchyma tissues are mainly terpenoids, flavonoids and other phenolic compounds; young and expanded leaves are equally rich in metabolites. Thus, the information obtained in this study is essential for conducting chemotaxonomic studies in this group, helping to promote selective collection and conservation of species.     

Secondary metabolites in plants: transport and self-tolerance mechanisms

Plants produce a host of secondary metabolites with a wide range of biological activities, including potential toxicity to eukaryotic cells. Plants generally manage these compounds by transport to the apoplast or specific organelles such as the vacuole, or other self-tolerance mechanisms. For efficient production of such bioactive compounds in plants or microbes, transport and self-tolerance mechanisms should function cooperatively with the corresponding biosynthetic enzymes. Intensive studies have identified and characterized the proteins responsible for transport and self-tolerance. In particular, many transporters have been isolated and their physiological functions have been proposed. This review describes recent progress in studies of transport and self-tolerance and provides an updated inventory of transporters according to their substrates. Application of such knowledge to synthetic biology might enable efficient production of valuable secondary metabolites in the future.     

培菌昆虫相关放线菌的次级代谢产物研究进展

昆虫共生微生物是一种特殊的微生物资源,其中放线菌在昆虫肠道、体表和巢穴中广泛分布。近年来,人们从培菌昆虫来源的放线菌中分离得到多种新型化合物,可以选择性抑制菌圃的致病真菌,部分还对植物致病真菌、昆虫致病真菌、人类病原菌和癌细胞有抑制活性。因此,研究培菌昆虫相关微生物不仅有助于了解宿主与微生物的共生机制,还能发掘新的活性物质,用于生物农药、生物医药的开发。本文对培菌昆虫来源放线菌次级代谢产物的研究进展进行了综述。     

Development of an Efficient Protocol of RNA Isolation from Recalcitrant Tree Tissues

Isolation of RNA from recalcitrant tree tissues has been problematic due to large amounts of secondary metabolites and interfering compounds in their cells. We have developed an efficient RNA extraction method, which yielded high-quality RNA preparations from tissues of the lychee tree. The method reported here utilized EDTA, LSS, and CTAB to successfully inhibit RNase activities. It was found that a high ionic strength brought about by 2 M NaCl was necessary. In addition, secondary metabolites and other interfering compounds were effectively removed using sodium borate and PVPP under a deoxidized condition. The quality of purified RNA was tested by both RACE and Northern blotting analysis, ensuring that the RNA could be used for subsequent gene expression analysis. This method has been successfully applied to purify RNA from 15 other plant species. In conclusion, the protocol reported here is expected to have excellent applications for RNA isolation from recalcitrant plant tissues.     

Adventitious Roots and Secondary Metabolism

Plants are a rich source of valuable secondary metabolites and in the recent years plant cell, tissue and organ cultures have been developed as an important alternative sources for the production of these compounds. Adventitious roots have been successfully induced in many plant species and cultured for the production of high value secondary metabolites of pharmaceutical, nutraceutical and industrial importance. Adoption of elicitation methods have shown improved synthesis of secondary metabolites in adventitious root cultures. Development of large-scale culture methods using bioreactors has opened up feasibilities of production of secondary metabolites at the industrial levels. In the present review we summarize the progress made in recent past in the area of adventitious root cultures for the production of secondary metabolites.     

Adventitious Roots and Secondary Metabolism

Plants are a rich source of valuable secondary metabolites and in the recent years plant cell, tissue and organ cultures have been developed as an important alternative sources for the production of these compounds. Adventitious roots have been successfully induced in many plant species and cultured for the production of high- value secondary metabolites of pharmaceutical, nutraceutical and industrial importance. Adoption of elicitation methods have shown improved synthesis of secondary metabolites in adventitious root cultures. Development of large-scale culture methods using bioreactors has opened up feasibilities of production of secondary metabolites at the industrial levels. In the present review we summarize the progress made in recent past in the area of adventitious root cultures for the production of secondary metabolites.     

Secondary chemicals protect mould from fungivory

The vast repertoire of toxic fungal secondary metabolites has long been assumed to have an evolved protective role against fungivory. It still remains elusive, however, whether fungi contain these compounds as an anti-predator adaptation. We demonstrate that loss of secondary metabolites in the soil mould Aspergillus nidulans causes, under the attack of the fungivorous springtail Folsomia candida, a disadvantage to the fungus. Springtails exhibited a distinct preference for feeding on a mutant deleted for LaeA, a global regulator of Aspergillus secondary metabolites. Consumption of the mutant yielded a reproductive advantage to the arthropod but detrimental effects on fungal biomass compared with a wild-type fungus capable of producing the entire arsenal of secondary metabolites. Our results demonstrate that fungal secondary metabolites shape food choice behaviour, can affect population dynamics of fungivores, and suggest that fungivores may provide a selective force favouring secondary metabolites synthesis in fungi.     

Diethylthiophosphate and diethyldithiophosphate induce genotoxicity in hepatic cell lines when activated by further biotransformation via Cytochrome P450

Organophosphorous (OP) compounds are the most commonly used pesticides. There are several published reports on the direct toxicity of OP pesticides, but few data on the toxicity of their metabolites. To determine if diethylthiophosphate (DETP) and diethyldithiophosphate (DEDTP), two of the major OP metabolites, demonstrate genotoxicity, and to elucidate the possible genotoxic mechanisms, we treated WRL68, HepG2, HeLa and human blood cells with different concentrations of DETP and DEDTP. We evaluated the possible contribution of oxidative stress generation and P450 enzymes to the genotoxicity of the OP metabolites, as determined using the comet assay. Our results showed that both OP metabolites (DETP and DEDTP) induce DNA damage only in the hepatic cell lines, and this effect could be related to a secondary non-diffusible metabolite generated by the activity of P450 enzymes since P450 enzyme inhibitors also inhibited the induction of DNA damage in hepatic cells. These secondary metabolites should be taken into account when assessing risk to human populations exposed to OP pesticides.     

Hybrid isoprenoid secondary metabolite production in terrestrial and marine actinomycetes

Terpenoids are among the most ubiquitous and diverse secondary metabolites observed in nature. Although actinomycete bacteria are one of the primary sources of microbially derived secondary metabolites, they rarely produce compounds in this biosynthetic class. The terpenoid secondary metabolites that have been discovered from actinomycetes are often in the form of biosynthetic hybrids called hybrid isoprenoids (HIs). HIs include significant structural diversity and biological activity and thus are important targets for natural product discovery. Recent screening of marine actinomycetes has led to the discovery of a new lineage that is enriched in the production of biologically active HI secondary metabolites. These strains represent a promising resource for natural product discovery and provide unique opportunities to study the evolutionary history and ecological functions of an unusual group of secondary metabolites.     

Eliciting secondary metabolism in plant cell cultures

Plant cell cultures are potentially rich sources of valuable pharmaceuticals and other biologically active phytochemicals, but relatively few cultures synthesize secondary compounds over extended periods in amounts comparable to those found in the whole plant. Frequently, no secondary metabolites characteristic of the intact plant are produced. So far, the manipulation of culture media, culture conditions and phytohormone levels have, in general, failed to permit commercial production of those phytochemicals useful in medicine and industry. This almost certainly reflects the lack of understanding of basic secondary metabolic regulation in cultured plant cells.

Microbial insult can induce antibiotic phytochemical synthesis in cultured plant cells: the microbial molecules which stimulate synthesis have been called ‘elicitors’. Increased synthesis of secondary products in response to elicitation of various types appear to be the general response of cultured cells. This paper illustrates the immense biotechnological potential of plant cell culture—‘elicitor’ (inducer) interactions to the large scale production of secondary metabolites, and suggests several lines of enquiry that remain to be authoritatively treated.     

Role of Secondary Metabolites and Brassinosteroids in Plant Defense Against Environmental Stresses

Being sessile, plants are subjected to a diverse array of environmental stresses during their life span. Exposure of plants to environmental stresses results in the generation of reactive oxygen species (ROS). These activated oxygen species tend to oxidize various cellular biomolecules like proteins, nucleic acids, and lipids, a process that challenges the core existence of the cell. To prevent the accumulation of these ROS and to sustain their own survival, plants have developed an intricate antioxidative defence system. The antioxidative defence system comprises various enzymatic and nonenzymatic molecules, produced to counter the adverse effect of environmental stresses. A sizable number of these molecules belong to the category of compounds called secondary metabolites. Secondary metabolites are organic compounds that are not directly involved in the growth and development of plants but perform specialized functions under a given set of conditions. Absence of secondary metabolites results in long-term impairment of the plant’s survivability. Such compounds generally include pigments, phenolics, and so on. Plant phenolic compounds such as flavonoids and lignin precursors have been reported to accumulate in response to various biotic and abiotic stresses and are regarded as crucial defence compounds that can scavenge harmful ROS. Another important category of plant metabolites, called brassinosteroids, exhibit stress regulatory and growth-promoting activity and are classified as phytohormones. Elucidation of the physiological and molecular effects of secondary metabolites and brassinosteroids have catapulted them as highly promising and environment-friendly natural substances, suitable for wider application in plant protection and crop yield promotion. The present review focuses on our current understanding of how plants respond to the generation of excessive ROS and the role of secondary metabolites and brassinosteroids in countering the adverse effects of environmental stresses.     

Redox active secondary metabolites

Various secondary metabolites from plants, bacteria and fungi are redox active and able to modulate the intracellular redox equilibrium in living cells. Many of these compounds behave as antioxidants, yet some of them also cause oxidative modifications, which may ultimately result in cell death. Natural isothiocyanates and xanthohumol, for instance, appear to act specifically in and against cells with a disturbed redox balance, such as certain cancer cells. Similarly, polysulfane and pyocyanin derivatives employ the glutathione antioxidant defense system of cells to generate a lethal cocktail of reactive oxygen species. Together, these redox-modulating metabolites provide promising new leads to target selectively certain cancer cells. They may also be useful in the treatment of autoinflammatory diseases.